1. Field of the Invention
The invention relates to line-narrowed excimer and molecular fluorine lasers, and particularly to a technique for testing a linewidth narrowing and/or wavelength selection and wavelength tuning module of such a line-narrowed laser.
2. Discussion of the Related Art
A line-narrowed excimer or molecular fluorine laser used for microlithography provides an output beam with specified narrow spectral linewidth. It is desired that parameters of this output beam such as wavelength, linewidth, and energy and energy dose stabilty be reliable and consistent. Narrowing of the linewidth is generally achieved through the use of a linewidth narrowing and/or wavelength selection and wavelength tuning module (hereinafter xe2x80x9cwavelength selectorxe2x80x9d) consisting most commonly of prisms, diffraction gratings and, in some cases, optical etalons. The wavelength selector typically functions to disperse incoming light angularly such that light rays of the beam with different wavelengths are reflected at different angles. Only those rays fitting into a certain xe2x80x9cacceptancexe2x80x9d angle of the resonator undergo further amplification, and eventually contribute to the output of the laser system. Parameters of the wavelength selector such as the magnitude of angular dispersion, reflectivity for specific wavelengths, linearity (i.e. absence of wavefront distortions), scattering of the beam, etc., will thus affect the performance of the laser. Therefore, it is desired to avoid or minimize any variability in the quality of wavelength selector modules when such lasers are mass-produced.
In a conventional quality control procedure, a wavelength selector is first installed into the laser system, and then the performance of the laser is evaluated. The problem with this approach is that the performance of the laser is a complex function of many components, not only of the wavelength selector. Therefore, poor performance of the laser usually cannot be unambiguously traced to the wavelength selector. Additionally, even if it is determined that the wavelength selector is the cause of substandard laser performance, it is generally not easy to identify which parameter of the wavelength selector in particular presents a problem.
Wavelength selectors vary in their response to the exposure to high power laser beams that cause heating and aging of the optical components. For example, nonuniform heating of the optical elements of the wavelength selector may substantially degrade their quality by, for example, distorting the wavefront of the retroreflected beam. Again, the quality of the optical components, as well as the assembly of the components (which may cause mechanical stress, for example), determines the behavior of the wavelength selector under irradiation conditions encountered in a laser. It is desired to have a technique for observation of such radiation-induced distortions in real time and under controlled conditions.
It is a first object of the invention to provide a quality testing technique for minimizing any variability in the quality of line-narrowing and/or selection modules, or wavelength selectors, particularly for excimer or molecular fluorine lasers, and also for any line-narrowed laser system.
It is a second object of the invention to provide a quality testing technique in accord with the first object that is performed in real time and under controlled conditions.
A method is therefore provided in accord with the above objects for testing the quality of a line narrowing and/or wavelength selection/tuning module that has been assembled for use with a line-narrowed laser. The method includes providing a test beam which has been previously line-narrowed using an installed line-narrowing and/or wavelength selection/tuning module. Then, the test beam is directed into the test module. Next, the one or more properties of the retroreflected beam are measured, i.e., after the beam has traversed the test module. The quality of the test module and one or more of its components may be determined based on the measurements. Such properties as wavefront distortions, excessive scattering, total reflectivity, total dispersion and aging of components of, from or due to the test module are measured for making this quality determination. Advantageously, the method allows measurement of the xe2x80x9capparatus functionxe2x80x9d of the line-narrowing and/or wavelength selection module independently of the laser into which it is to be installed, thus giving a precise indication of the quality of the module and which properties or parameters, if any, are substandard.
An apparatus for testing the quality of a test linewidth narrowing and/or wavelength selection module that has been assembled for use with a line-narrowed laser includes the test module, a test laser having an installed line-narrowing and/or wavelength selection module for providing a line-narrowed test laser beam to be directed into the test module, and an observation device such as a detector for measuring one or more properties of the beam after the beam traverses the test module. A processor is preferably further provided for determining the quality of the test module based on the one or more measured properties.